Flathead Lake
Lake County, MontanaView mapDirections
Not Rated: one monitoring station are on record, but they measured water clarity only. LakeQuality’s A–F method grades total phosphorus, total nitrogen, chlorophyll-a, and turbidity; clarity is shown, not scored.
- Public access
- Limited data
Flathead Lake in Lake County, Montana is not rated for water quality — its monitoring record does not meet the evidence bar. Water clarity: 11.8 ft visibility.
- Clarity
- 11.8 ft
- Public access
- Yes
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Watch Flathead Lake’s letter
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Why Flathead Lake has no grade
Flathead Lake has monitoring on record from one station, but only water clarity was measured. LakeQuality’s A–F letter uses total phosphorus, total nitrogen, chlorophyll-a, and turbidity against EPA National Lakes Assessment condition classes. With none of those grading measurements on record, Flathead Lake is Not Rated — not an F. Its 11.8-foot median from 2 readings collected 2012 is still shown as clarity, but clarity alone cannot earn a letter.
Not Rated is EPA Integrated Reporting Category 3 — insufficient information. Everything below that has been measured is still reported on this page.
Sampling is run by Montana DEQ and volunteer programs, and published through the EPA Water Quality Portal. Read the full grading methodology.
Flathead Lake is a lake in Lake County, Montana. Limited fish-species data available. Public access is available with 2 public boat landings.
Source: EPA Water Quality Portal sampling records, Montana DEQ, last sampled 2012-07-11. Grade methodology: LakeQuality methodology.
What Flathead Lake is good for
From access and monitoring records, not editorial picks.
- Clear water11.8 ft of visibility
- Boat launch2 documented ramps
Swimming Safety
Not ratedLimited data
The letter is not a swimming clearance. Check local advisories before you swim.
Water quality measurements for Flathead Lake
These readings exist but are not enough for an A–F grade — see why.
Good clarity, visible to about 11.8 ft. TSI 42.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 11.8 ft | Not graded |
| Phosphorus | No recent samples | |
| Chlorophyll-a (Algae) | No recent samples | |
| Trophic State Index (TSI) | 42 | Mesotrophic |
Moderate nutrients, good water quality
→ Flathead Lake water level: the live USGS gauge reading
Recreation & Access
- Rollins Lakeshore DrivePublic
- Park LanePublic
Named access points in the USGS National Boat Ramps inventory, within about 3 miles. More lakes with boat ramps
Fishing Flathead Lake
Graded lakes near Flathead Lake
Flathead Lake has no grade, but these lakes within a short drive do.
Nearby Lakes in Lake County
Frequently asked questions about Flathead Lake
Where is Flathead Lake?
- Flathead Lake is a lake in Lake County, Montana. LakeQuality has not rated this lake (UNRATED) — no nutrient or algae measurement meets the evidence bar (at least 3 samples within 10 years).
What is the water quality grade for Flathead Lake?
- Flathead Lake in Lake County, Montana is not rated for water quality — its monitoring record does not meet the evidence bar. Water clarity: 11.8 ft visibility.
Is Flathead Lake safe to swim in?
- The letter is not a swimming clearance. Check local advisories before you swim.
Are there boat ramps on Flathead Lake?
- Flathead Lake has 2 documented boat landings in the USGS National Boat Ramps inventory. Named access points include Rollins Lakeshore Drive, Park Lane. Public access is listed as yes.
Explore Flathead Lake across LakeQuality
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Where this data comes from
2 measurements from 1 monitoring station, 2012. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- North American Lake Management Society
NALMS2 samples · 2012
Monitoring station identifiers (1)
Grade thresholds from the EPA National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See how grades are calculated and data sources.
Other records on this page
Named boat-access points from the USGS National Boat Ramps inventory